Chemical solder comprising a metal salt, polyphthalaldehyde and a solvent

ABSTRACT

A chemical solder is described that includes an organometallic which thermally degrades within a predetermined temperature range to a metal and volatile compounds. The solder also includes a polymeric matrix that decomposes within the same temperature range to volatile fractions, thereby leaving only the metal. 
     A method for bonding first and second bodies is disclosed wherein a chemical solder, as above-described, is disposed between the first and second bodies and heat is applied to elevate the solder to the predetermined temperature range to thermally degrade the organometallic compound and to decompose the polymeric matrix. The remaining metal bonds the first and second bodies.

This is a continuation of copending application Ser. No. 07/538,288filed on Jun. 14, 1990, now abandoned.

FIELD OF THE INVENTION

This invention relates to organometallic polymers, and more particularlyto certain organometallic polymers which lend themselves to the creationof solder-like bonds between metallic bodies.

BACKGROUND OF THE INVENTION

Presently, technologies for connecting semiconductor chips to underlyingconductor arrays include thermo-compression and solder-based bonding.Thermo-compression bonding is a low cost process and is used for manylow-to-mid performance chip applications. It requires the application ofsignificant pressure to form the chip/conductor interconnection and may,at times, damage underlying active chip structures. Generally, thisbonding method is not usable with chips which have active circuitrybeneath the bonding pads. Its use with chips having perimeter I/O andarea I/O interconnections significantly reduces the area of siliconavailable to accommodate circuitry and bonding pads and increases thechip's cost. Solder-based technologies have been used to interconnect tosuch chip structures, as they provide low stress bonding methods andenable circuitry to be located under the bonding pads. However,solder-based technologies are relatively expensive and typically havebeen limited to high-end packaging applications.

Efforts have been expended to establish chip connection by writingconnector lines using thick film pastes similar to those used inhybrid-circuit technology. These pastes are typically suspensions of40%-80% metal flakes in a polymer, which polymer is cured (i.e., notvolatilized) to form metallic lines. While it is preferred to producemetallic lines whose resistivity approaches that of the pure metal,because some of the paste remains in the metallic lines, substantiallyhigher resistivities are found.

Another prior art technique for providing conductive circuit linesemploys chemical vapor deposition of a volatile organometallic complexon a substrate through the use of a laser to "write" lines on thesubstrate. Since this procedure is a direct "write" technique, it isthroughput limited and does not lend itself to high volume production.Circuit lines had also been written with lasers by reducing a complex ina thermoplastic polymer. In none of these cases is a direct bond formed.Each is employed to create circuit lines either by laser deposition orby fine line printing of thick film pastes. These efforts are covered inmore detail in the following references:

A. Auerbach, "Method for Reducing Metal Salts Complexed in a PolymerHost with a Laser," J. Electro. Chem. Soc., 132, 6, p. 1437, 1985.

M. Ohuchi et al., "Planar LSI Interconnection Method Utilizing PolymericConductor", IMC 1986 Proceedings, Kobe, May 28-36, 1986.

Auerbach, "A New Scheme for Device Packaging", IEEE Transactions onComponents, etc, Vol. CHMT-8, No. 3, Sep. 1985, pp. 309-312

Hsu et al., "The Wire Bondability of Thick Film Gold, Sputtered ThinFilm Gold and Metallo-Organic Thin Film Gold", Proceedings 1985International Symposium on Microelectronics, Int. Soc. HybridMicroelectronics, Anaheim, Calif., 11-15 Nov. 1985, pp. 428-434.

C. Needes et al., "Thick Film Materials for Copper Hybrid Circuits", IMC1986 Proceedings, Kobe, May 28-30, 1986.

F. Houle, C. Jones, T. Baum, C. Pico, C. A. Kovac, "Laser Chemical VaporDeposition of Copper", Appl. Phys. Lett., 46, p. 204, 1985.

T. Baum and C. Jones, "Laser Chemical Vapor Deposition of Gold", Appl.Phys. Lett. 47, p. 538, 1985.

Certain aspects of the above-noted deposition techniques presentdifficulties when applied to semiconductor applications. For instance,in order to drive off the pastes used to carry the metal flakes, hightemperatures must be employed. These temperatures tend to damage andotherwise unfavorably affect circuit components, while still leaving theorganic material incorporated in the final conductive lines. It is knownthat certain organometallics reduce to a metal state at much lowertemperatures than paste/polymer combinations (e.g. on the order of100-300° C. as contrasted to 800° C.-500° C.). Organometallics have beenused in the prior art to create metal seeding layers for subsequentlyapplied conductor patterns. For instance, in U.S. Pat. No. 4,574,095 toBaum et al., a palladium organometallic compound is irradiatedselectively by light, thereby depositing palladium seeds in theirradiated areas. Following the deposition of the palladium seeds,copper is deposited thereon.

In U.S. Pat. 4,701,351 to Jackson, a thin organometallic layer isapplied as a coating and then may have an additional coating of aphotoresist placed thereover. During subsequent processing, portions ofthe organometallic are reduced to the metal state, to provide asubstrate upon which subsequent conductor deposition can occur.

In U.S. Pat. No. 4,734,481 to Steinmann, a class of organometallicpolymers is described, particularly useful for photoactive coatingagents for various types of substrates. The preferred organometallicpolymer is an end-capped polyphthalaldehyde. These materials areemployed similarly to other photosensitive photomicrography materials.In other words, they are applied as a layer, imaged in a conventionalmanner and the irradiated areas decomposed, with resultant monomersevaporating and exposing the underneath substrate. A benefit derivingfrom the use of these organometallics is that they convert to theirvolatile monomeric state at relatively low processing temperatures andleave little or no residue.

It is therefore an object of this invention to provide aninterconnection technique which employs low temperatures and providesgood metallurgical results.

It is another object of this invention to provide a low temperatureinterconnection technique which enables the generation of a homogenousmetallic bonds.

It is a further object of this invention to provide a method of formingmetal-metal bonds which are characterized by low stress, low temperatureconditions.

SUMMARY OF THE INVENTION

A chemical solder is described that includes an organometallic whichthermally degrades within a predetermined temperature range to a metaland volatile compounds. The solder also includes a polymeric matrix thatdecomposes within the same temperature range to volatile fractions,thereby leaving only the metal.

A method for bonding first and second bodies is disclosed wherein achemical solder, as above-described, is disposed between the first andsecond bodies and heat is applied to elevate the solder to thepredetermined temperature range to thermally degrade the organometalliccompound and to decompose the polymeric matrix. The remaining metalbonds the first and second bodies.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a semiconductor chip/lead assembly whereina ball of chemical solder has been applied to a land area of the semiconductor chip.

FIG. 2 is a section of the structure shown in FIG. 1 wherein thechemical solder has been preapplied to the lead rather than the landarea on the chip.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the chemical solder of this invention includesa selected organometallic complex which is blended with a solvent and apolymer into a paste. Suitable organometallic complexes are those thatwill thermally reduce to a pure metal along with volatile ligands.Examples of such complexes include: copper acetylacetonate, cupricacetate, palladium acetylacetonate, platinum acetylacetonate, anddimethyl gold acetylacetonate. Each of these organometallic complexes,when heated, reduces to the pure metal state along with volatilefractions. Each of those complexes completes the reduction phase in atemperature range which is less than approximately 325° C. Preferably,the organometallic complex should contain a metal selected from theclass of gold, silver, tungsten, nickel, aluminum, copper, palladium, orplatinum.

A suitable polymer for use with the chemical solder must have a highionic purity and decompose at relatively low temperatures (i.e., lessthan 325° C.) to volatile, non corrosive oligomers, or fragment intovolatile fractions. The term decompose will, hereinafter, be used hereinto include both the depolymerization and fragmentation reactions. Thefact that such polymers decompose into non-corrosive, volatilecomponents eliminates contaminating residues and the need for cleaning.It is to be understood that a small percentage of polymeric residue canbe accommodated in the solder, however no more than 10% residue isacceptable. The level of residue can be determined by measuring theresistivity of the remaining metal after the application of heat.Essentially, it is most desirable to obtain resistivities as close aspossible to the bulk metal.

The preferred polymer for use with the chemical solder ispolyphthalaldehyde. Other polymers which are acceptable are:

Poly (isopropenyl ketones)

e.g., poly (isopropenyl t-butyl ketone;

Poly (alpha-methyl styrenes),

e.g., poly (p-hydroxy-alpha-methyl styrene);

Poly (t-butyl carbonates);

Poly (alkene oxides);

Poly (alkene sulfides);

e.g., poly (styrene oxide), poly (styrene sulfide);

Alternating copolymers of phthalic anhydride with epoxides; or

Poly (methacrylates),

e.g. poly (methyl methacrylate), poly (benzyl methacrylate).

Suitable solvents for use with the chemical solder of this inventioninclude:

Polar protic solvents,

e.g., Dimethyl formamide, N,N-Dimethyl acetamide,N-methyl-2-pyrrolidinone;

Aromatic hydrocarbons,

e.g., Toluene, Xylene;

or

Glyme;

Diglyme;

Gamma-butyrolactone;

(+/-) Propylene carbonate;

Propylene glycol methyl ether acetate;

Methyl ethyl ketone;

methyl iso-butyl ketone.

Polymers usable with this invention each exhibit apolymerization/depolymerization reaction which is reversible As heat isapplied, the reaction equilibrium shifts towards the monomer state, withthe monomer being volatile at the elevated temperature. In some cases,the depolymerization reaction is not back to the monomer state, butresults in the polymer decomposing into oligomeric fractions which are,in themselves volatile. It is important in each case, that the result ofthe heating phase lead to a volatile compound or fraction which isdriven off at the elevated temperature. It has been observed withpolyphthalaldehyde, that its decomposition commences at approximately130° C. and at 190° C., little or none is left.

As regards the organometallic reaction, at the elevated temperature theorganometallic reacts to give zero valence metal with the organicportion volatilizing. The remaining metal forms clusters and finallyagglomerates to form a continuous metal deposit. It has been noted withdimethyl gold acetylacetonate, that the decomposition of theorganometallic commences at between 130° to 150° C. and is complete atbetween 200° C.-300° C.

Reductive decomposition of the organometallic complex to the zero valentmetal may be further facilitated by exposure of the complex to areducing gas during heating. This can be accomplished through the use ofa bonding thermode which provides a uniform gas flow about the complexduring heating. A reducing or forming gas of from 3% to 10% H₂ in N₂ ispreferred.

Referring now to FIG. 1, there is shown a semiconductor chip 10 which isprovided with a conductive land area 12 that is, in turn, surrounded bya passivation layer 14. A deposit of chemical solder 16 is emplaced onland area 12 and a conductive lead 18 has both heat and a modest amountof pressure applied, causing it to come into contact with chemicalsolder deposit 16. As the entire combination is brought up intemperature, the polymeric components within chemical solder 16decompose, volatilize, and leave a metal deposit which bonds conductor18 to land area 12.

In FIG. 2, a substantially identical structure is shown except thatchemical solder deposit 16 has been preapplied to conductor 18 ratherthan the land area 12. When both heat and modest pressure are appliedbetween conductor 18 and land area 12, a bonding occurs there between assoon as the decomposition polymer products have volatilized.

Application of the chemical solder can be accomplished by dispensing orby conventional printing techniques through a mask or a screen.Lithographic methods can also be used by the incorporation of aphotosensitizer into the chemical solder. Acceptable photosensitizersare: thioxanthones, polyaromatic hydrocarbons (anthracenes), cationic orradical initiators, sulfonium salts or esters. The inclusion of aphotosensitizer allows the blanket application of the chemical solder,followed by masking, exposure and development of the photoresistmaterial to leave a desired pattern. Once the solvent is driven off, thematerial is stable and can be stored before further use.

EXAMPLE

A chemical solder has created interconnections between the lands of asemiconductor chip and the inner leads of a tape automated bonding (TAB)package by the following method -- a one:one mixture ofpolyphthalaldehyde and DMF was blended with dimethyl goldacetylacetonate to give a 20% loading of the organometallic complex inthe polymer. The resulting paste was then applied along 130 input/outputpads of a semiconductor chip. The inner leads of a TAB frame werealigned over the pads of the semiconductor chip, placed in contacttherewith and heated at 300° C. The resulting interconnects had averagepull strengths of 80 grams, the highest pull strengths being observed at121 grams. This is in contrast to typical TAB thermo-compression bondstrengths of 70 grams and typical wire bond strengths of 12 grams.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. Accordingly, the present invention is intended to embrace allsuch alternatives, modifications and variances which fall within thescope of the appended claims.

We claim:
 1. A chemical solder which, upon application of heat within apredetermined temperature range having an upper limit of 325° C.,degrades and decomposes to leave a metal, said solder comprising:acompound selected from the class consisting of: copper acetylacetonate,cupric acetate, palladium acetylacetonate, platinum acetylacetonate,dimethyl gold acetylacetonate, said compound thermally degrading to ametal and volatile components within said predetermined, elevatedtemperature range, said compound producing, after degradation, a depositof said metal for bonding adjoining metal conductors; a polymeric matrixof poly phthalaldehyde which decomposes to volatile fractions withinsaid predetermined, elevated temperature range; and a suitable organicsolvent.
 2. The chemical solder of claim 1 further comprising aphotosensitizer.
 3. The chemical solder of claim 2 wherein saidphotosensitizer is selected from the class consisting of: tioxanthonesm,polyaromatic hydrocarbons (anthracenes), cationic initiators, radicalinitiators, sulfonium salts, esters.
 4. The chemical solder as recitedin claim 1 wherein said compound contains a metal selected from thegroup consisting of Au, Ag, W, Ni, Al, Cu, Pd, and Pt.